There has conventionally been known various methods for forming a ceramic porous membrane on a porous substrate. For example, a hot coat method is a method where a porous membrane is formed by application on the outer surface of a heated tube substrate by the use of fabric containing silica sol with rubbing the fabric against the tube substrate (Non-Patent Document 1)
There has been known also a method where a porous membrane is formed by filtration formation on the inner surface of a porous substrate of a monolith shape of a tube shape or a circular cylindrical lotus root shape (see Patent Documents 1 and 2), where a membrane is formed on the inner surface of the porous substrate by maintaining the pressure to be lower on the outer surface side than that on the inner surface side, where the sol is brought into contact, of the porous substrate.    Patent Document 1: JP-A-3-267129    Patent Document 2: JP-A-61-238315    Non-Patent Document 1: journal of Membrane Science 149 (1988) 127-135
However, the hot coat method has a problem that uniform membrane cannot be formed on the entire surface of the substrate and a problem that a membrane can be formed only on the outer surface of the tube. In addition, the method cannot be applied to a monolith type substrate. On the other hand, in the filtration membrane formation method, a solvent present in pores of the substrate may flow out toward the membrane side upon drying after forming the membrane to cause peeling of the membrane. As a result, there arises a problem that a porous membrane formed on the surface of the substrate after firing has a defect. In addition, though the dipping method can be applied to a monolith type substrate, the number of membrane formation operations is large.
The present invention aims to provide a ceramic filter which is formed by a small number of membrane formation operations, has few defects, has a thin and uniform membrane thickness, and high resolution.